I figured it out while doing a games jam a few months back. If you open the layer manager (it’s a few buttons over from transform tools) in 3ds max at the bottom where it says “Layer Explorer”, to the right of that, select “Sort By Hierarchy”.
http://i.imgur.com/jQ8Abz2.png (Diagram)
Now you need to locate the triangle base you get with all cat rigs, drag the pelvis and any other helpers (like feet platforms) onto that, this might require you to re-skin the model, so keep that in mind. I’d then advise renaming the triangle platform to “Root”, I don’t believe that’s important though.
Now, if you want to do an attack animation or anything that requires root motion, make sure to only move the root helper, when exporting your fbx as well, make sure to hit the “convert deforming dummies to bones” or the cat rig will be converted to a mesh.
Hope this helps.